Copolymerization of tetrafluoroethylene and hexafluoropropylene



United States Patent CGPQLYNIERIZATIQN' GF TETRAFLUORGETHYL- ENE AND HEXAFLUDRGPRORYLENE Maurice Joseph Couture, Parkersburg, W. Va, Darrel'Lee Schindler, @rarzge, Tenn, and Robert Bruce Weiser, Parire'rsburg, W. .Va., assignors ME. I. du Pont de Nernours and'ComPflBY, ,Wigton, Del a corporation tit-Delaware No Drawing. Fiicd July 6, 1961, Ser. No. 122,078

I Qiairns. (Cl. 260-873) 2 they are more readily melt fabricable. Because of the in creasing commercial importance of these copolymers, a need has arisen for a more satisfactory and economical method of'rnanufacture.

It isan object of the present invention to provide an improved method for the preparation of copolymers of tetrafiuoroethylene and hexaiiuoropropylene. It is a further object to prepare these copolyrners in the form of aqueous dispersionshavingincreased solids contents. Astill further object is to produce these copolymers at rates which are faster than those realized inthe procedures described by the prior art. ;Another object of the present invention is to provide a process which yields tetrafluoroethylene hexafluoropropylene copolymers which have useful molecular weights and compositions. A still further object is to improve the eiiiciency of :the copolymerization so that the amount of unconverted comonomersfwhich must be recycled is substantially reduced.

Other objects will become apparent hereinafter.

The objects of the invention are achieved byconducting thepolymerization of the mixed comonomers, tetrafluoroet'hylene and hexafluoropropylene, at 95 to 138 C. in an aqueous system in the presence of a dispersing agent and using a freeradical polymerization initiator, with the vapor space density of' the mixed comonomers being maintained at 0.18 to 0.30 grams/cc.

In most copolymerizations'composition is determined by the ratio and reactivities of comonomers in the reactant 3 mixture, while molecular weight is controlledby such variables as temperature, pressure and type and concentration of initiator. A limiting factor in the preparation of polymers by dispersion techniques is the maximum concentration of polymer in dispersion which can be In order to minimize this coagulation tendency dispersing agents'frequently are employed. in the presentinvention it hasbeen discovered that. by employing an unusual combina-.

tion of temperature, pressure, comonomer mixture vapor space density, polymerization initiator, initiator concentration, initiator scheduling and dispersing agent concen traticn, it is possible to achieve a useful combination of polymerization rate, copolymer composition and copoly rner dispersion concentration. It further has been found that the properties of the products thus prepared are not completely consistent with the properties of the materials prepared by the methods known to the art. Theeffect of each of the Variables will be described. 7 7

Temperature is a recognized critical variable Iin free radical polymerizations. -Whereas it is known that increased temperatures normally provide increasedr'eaction rates, the temperature employed in the copolymer'ization Patented May 5, 1964 of tetrafiuoroethylene and hexafluoropropylene by means of the present invention has been found to exhibit an unusual influence on polymerization rate between about, and 138 C. At an operating total pressureof Q00 V p.s.i.g., for example, an optimum level is reached, asis shown in Table I, at about 119 (3., with variations of less than 10% from this optimum rate at between and 127 C. and less than 5%, at between 113 and 123 C. This unusual criticality of temperature has been heretofore unrecognized in prior art processes such as that described, for example, in US. Patent 2,946,763.

TABLE I Efiect of Temperature on Polymerization Rate of Hexafluoropropylene-Tetrafluoroethylene Mixtures Specific melt viscosity: 7.5 X 10 poises 0.1% dispersing agent 74 wt. percent hexafluoropropylene in comonomer mixture Vapor space density: 0.355 to 0.208 g./cc.

Pressure: 600 p.s.i.g.

- comonomer vapor space density, per. se, usually is not considered incopolym'erization processes. The conditions commonly are 'acljusted'with regard to temperature, .pres,,

sure and composition. of the comonomer mixture so 'as to produce the desired copolymer. Although it is recognized that these variables affect density; it has been found, in the present inventionthat. although polymerization rate, copolymer composition and molecularweight can be controlled by the mixedicomonomer vapor space density,

thelatter exhibits an unusual influence between about 0.18 and 0.30Ygram/cc and Whenxthe temperaturefis .maintained at about C.,"for example,iespecially:b e-

tweenl 0.20 and 0.25 gram/cc, with-the optimum level at about 0.22-0.23 gram/cc. tionship between relative reactionrate and comonomer vapor space' density at 120. C. in the preparation of. 00-, polymers having a specific melt viscosity of 7:.5x10

poises'and a specific LR. ratio (both described infra) of 3.5. w I

in TABLE II,

Efiect of Comor to mer Mixture Vapor Space Density or:

Polymerization Rate of Hexafluoropropylene-Tetrafluo roethy Zene "Mixtures Specific melt viscosity 7.5 X 10 poises Specific LR. 3.5 p

0.15 dispersingngent Pressure and composition of comonomer mixture: variable Temperature: 120 C.

. Relative reaction rate (at Vapor spa'ce density (g./ccj density of 0.22: 1)

Relative Reaction Rate Table II depicts" the rela-.

. 3 Useful copolymers of tetrafluoroethylene and hexafluoropropylene having melt viscosities in the range 1.5 l to 1.5 x poises and specific I.R. ratios from 1.5 to 6.0 can be prepared from 95 to 138 C. while maintaining the vapor space density inthe aforementioned range of 0.18 to 0.30g1'am/cc.

The range of pressures under which the present process is operated is, of course, limited by the aforementioned variables, namely, temperature and comonomer vapor space density and, also, comonomer mixture composition. The limits previously set for these variables may be met by maintaining the pressure at between 400 and 750 p.s.i.g., but at the preferred temperature range of 110 to 127 C., preferably between 550 and 625 p.s.i.g., with the optimum pressure being about 575-600 p.s.i.g. The rate thus may be seen to peak in the same manner, based on pressure, as it did for the aforementioned mixed comonomer vapor space density.

The range of mixed comonomer compositions, likewise, is determined essentially by the variables previously defined, but usually is such as to include 25 to 95 wt. percent hexafluoropropylene. It has been noted, however, that mass transfer will vary depending upon the composi tion. Particularly at high hexafluoropropylene contents,

therefore high vapor densities, mass transfer becomes increasingly diflicult. This effect usually is overcome by increasing agitation rates. I

A further feature of thisinvention is the unusual eifect of initiator concentration on polymerization rate. The concentration of polymerization initiator in dispersion polymerizations normally is controlled to determine polymer molecular weight rather than polymerization rate. In somecases where a required amount of initiator is employed to achieve a satisfactory molecular weight, the rate of polymerization may be slower than desired. It hasbeen discovered that by using programmed scheduling of an amount of initiator that is in excess of that required to achieve the goal product molecular weight, a substantial increase in rate may be realized. This is eifected by precharging initiator and hexafluoropropylene in order to obtain increased particlenucleation prior to the addition of tetrafluoroethylene. After nucleation is complete tet'rafluoroethylene and the standard amount of initiator are added continuously during the polymerization cycle In order to prevent the precharged initiator from adversely affecting product molecular weight it is essential that the excess be destroyed immediately after the nucleation stage to preclude carryover into the polymerization stage. Such a result is achieved by selecting a polymerization temperature at which the initiator half-life is extremely small. This temperature will, therefore, be determined by the initiator employed. Conversely, if a specific'temperature is chosen, an initiator having ashort half-life at this temperature must be selected. It has been found that the amount of precharged initiator necessary to provide suflicient nucleation to give the increased polymerization rates must exceed a minimum quantity. The minimum quantity generally will be be tween 6.5 and 7.5 wt. percent of the total added during the polymerization stage. Since the latter will vary with the molecular weight, the minimum weight percent will, of course, be higher than the above for the high molecular weight products. The maximum quantity is critical only in that active initiator must not carry over into the polymerization stage, but usually does not exceed 65 to 75 wt. percent of that added during polymerization. The quantity of initiator injected during the polymerization stage is such as to provide a free radial generation rate of 4X10 to 3 1O- moles per minute per liter of solution at reaction conditions. The polymerization initiator may be any water soluble, free radical-producing compound. such as a peroxide, persulfate, azo compound .and the like, which has a half-life at reaction condition of between 95 and.138 C. of less than 9.0 minutes and preferably. less than 2.0 minutes, with an optimum halflife of less than 1.0 minute. In general, the minimum halflife is determined by the dimensions and shape of the equipment and the volumes of initiator solutions which can be handled conveniently. Normally, the half-life should not be less than 0.1 minute at reaction conditions. In the present process it has been found advantageous to employ ammonium persulfate or potassium persulfate as initiators.

Dispersing agents commonly are employed to increase the non-coagulated solids content of polymers prepared by aqueous dispersion techniques.- In the present invention it has been discovered that the polymerization rate, the polymer composition and molecular weight may be controlled by means of added dispersing agents. Table III shows the effect of dispersing agent on the relative polymerization rate at 120 C. Table IV relates the effect of dispersing agent on rate to the copolymer composition at 120 C. From this it is evident that the addition of dispersing agent may be covered to achieve a higher specific I.R. ratio by holding the production rate constant or-to achieve a higher production rate while maintaining a constant specific I.R. ratio. Table V shows the increased range of molecular weights and compositions which are attainable using the dispersing agent, while. Table VI exemplifies the range of polymer production rates, compositions and comonomer vapor composi tions which may be covered with the addition of 0.1% dispersing agent. By comparison, the most useful copolymers prepared without the dispersing agent have limited production rates of about 100 grams per liter hour for the range of variables shown.

TABLE III Efiect of Dispersing Agent on Polymerization Rate of Hexafluoropropylene Tetrafluoroethylene Mixtures Specific melt viscosity: 7 .5 X 10 poises 7 wt. percent hexafluoropropylene in comonomer mixture Vapor space dens1ty: 0.235 g./cc.

Pressure: 600 p.s.i.g.

Temperature: 120 C.

Dispersmg agent: ammonium QH-hexadecafluorononanoate Relative reaction rate Wt. percent agent: .(at 0% agent=l) TABLE IV Efiect of Dispersing Agent on Hexafluoropropylene- Tetrafluoroethylene Copolymer Composition Specific melt viscosity :'7 .5 X 10 poises wt. percent hexafluoropropylene in comonomer mixture Vapor space density: 0.235 g. cc.

Pressure: 600 p.s.i.g;

Temperature: 120 C.

Dispersin'g agent: ammonium 9Hhexadecafluorononanoate Specific LR. ratio Rate At g./ At 200 g./ At 300 g./ liter-hour liter-hour liter-hour Wt; percent agent: 0. 3. 50 2. 92 0.05 3. t 3. 21 0.10. 4. 3. 50 0.15 4. 3. 77 0.20 4. 3. 98 0.25 4.15 0.30 4. 23 0. as 4.30 0. 40 4. 43 0.45

TABLE v cific LR. Ratio for Hexafluoropropylene-Tetmfluoroethylene Copolymers' 75.wt. percent hexafiuoropropylene in comonomer mixture Constant polymerization rate V I Pressure: 600 p.s.i.g. Temperature: 120 C. V Dispersing agent: ammonium 9H-hexadecafiuorononanoate TABLE VI Relationship of Mixed COIIQQfiOIllBI Vapor Composition, Specific LR. Ratio and Production Rate Specific melt viscosity: 7.5)( poises Pressure: 600 p.s.i.g.

. Temperature: 120 C.

0.1% dispersing agent Wt. percent hexafiucropropylene in vapro space Specific LR. ratio p as 3.5 3.2

Production rate (g. /liter-hour): 150 59. 6 68.6 58.5 77. 6 65. 0 86.5 71. 3

A further advantage arising from the use of dispersing Efie ct of Dispensing Agent on Melt Viscosity and Sp eagents is the increased efiiciency of conionomer utilization. Because of this increased utilization the amount of unconverted comonomers which must be recovered and recycled is reduced by about a factor of two. In the present invention this reduction is facilitated by lowering the amount of hexafluoropropylene in the comonomer mixture by about 10%. For example, the amount of hexafluoropropylene in a comonomer mixture which normallyfis required to achievefla specific I.R. ratio of 3.5 can be reduced from 78-86 wt. percent to 70-78 wt. percent. The dispersing agent used is"not critical in. the present invention although the amount employed may have to be varied to compensate 'for variations in water solubility, nature of the hydrophilic and hydrophobic groups and the like. In general, the dis pcrsing agent should have a solubility in waterof at least: 0.1 wt. percent at reaction conditions, and'preferably at least 0.5 wt. percent. Those which have been found very effective are those containing a hydrophilic group and a hydrophobic portion said latter being a" fiuoroalkyl group containing at least six carbon atoms, with each carbon'atom except the ones alpha and omega to the hydrophilicgroup, containing two fluorine atoms.

The alpha and'omega carbon atoms may contain either fluorine or hydrogen. These dispersing agents may bc'represented by the general formula B(CF (CI-I A where B is hydrogen or fluorine, n is an integer at least equal to five, m is zero or one, the sum of m and n is 1 at least equal to 6, and A is an ionic (cationic or anionic) 'hydrophilic, group. The hydrophilic group may be any 0 minute.

of the well known anionic or cationic hydrophilic groups commonly incorporated into surface active agents. For example'it may be a free carb-oxylic, phosphoric, phosphonic or sulfuric acid group or the alkali metal, ammonium or substituted ammonium salt thereof, or an amine or substituted amino group; These dispersingagentsare described more fully in U.S. Patent 2,559,752. Examples of salts of the fluoro-alkylcarboxylic acids, include the ammonium and alkali metal salts of 7H-dodccafiuoroheptanoic acid, 8H-tetra-decafiuorooctanoic acid, 9H- hcxadecafluorononanoic acid, 1-1H-eicosafiuoroundecanoic acid, as well as the ammonium and alkali metal salts of the perfluorinated alk anoic acids. Ropresent-ativeof the other types of dispersing agents, supra, are ammonium 1,1,7H-dodecafluoroheptyl acid phosphate, ammonium 1,1,9H-hexadeoafluorononyl acid phosphate, 1,1,7H-

dodccafiuoroheptyl dihydrogen phosphate, 1,1,9H-hexadecafluorononyl sul liate, sodium 1,1H-heptadecafiuorononyl sulfate, GH-dodecafludrohexyl-phosphonic acid, ammonium 8H-hexadecailuorooctyl phosphonat'e, 1,1,91-1- hexadecaiiuorononylamine sulfate, and the like. The dispersing agent may also be formed in situ by precharging only 5' to 15% of the tetrafiuoroethylene usually added immediately afterthe nucleation stage. After-5 to 15 minutes with polymerization stage rate of initiator addition topermit formation of the dispersing agent the tetrafluoroethylene charging is continued and the polymerization is completed in the usual manner.

By the term specific melt viscosity as used herein, is meant the apparcntmelt viscosity as measured at 380 C. undera shearstress or 6.5 pounds per squareinch. The 'values herein referred 'to are determined using a melt indexer of the type described in American Society of Testing Materials test D-l238-57T, modified for cor-' rosion resistance to embody a cylinder and orifice of .Ampco aluminum bronze and a piston Weighing 60 grams, having a Stellite cobalt-chromium-tungsten all'oy tip. The resin is charged tothe 0.375 inch I.D. cylinder which is held at 380 C.:':0.5 C., allowed to'come .to an equilibriumtemperatureduring 5 minutes, and

extruded through the 0.0825 inch diameter, 0.315 inch long orifice under a piston loading of 5000 grams. The specific melt viscosity in poises is calculated as 53,150 divided by the observed extrusion rate ingrams per The term specific LR. ratio as used'herein refers to the net absorbance in the infra-red at a wave length of 10.18 micronsdivided by the net absorbance inthe in; fra-red at a wave length of 4.25 microns, of a film about 2 mils'thick obtained by melting, pressing and'water quenching a. sample of theresin. The valuesherein referred to are determined on film prepared by melting a 0.5 gram sample of the resin at 340 C., pressing the melted sample during one minute at 340 C. under 40,000 pounds force between the shiny sides of 5 square aluminum sheets having a combined thickness of 3 mils in a 6." square, 5 mil deep cavity of a mold assembly-consisting. of superposed fiat platens sepa'rated'by a centrally apertured spaced sheet, quenching'the mold assembly in ice water, and dissolving the adherent aluminum foil away fromthe .resultingfilm in hot (l00 C.) aqueous, 10 percent sodium hydroxide solution. A clear por- 'The net absorbances at; 10.18 and 4.25 microns are i measured in the conventional way. From correlations with material balances during resin synthesis and analyses of the products of controlled resin decompositions, it has benindicated that the specific LR. ratio when multiplied by 4.5 is numerically equal to the weight percent of combined hexafiuoropropylene contained in the resin.

The following examples are given to illustrate but not limit the various features of the present invention.

SUMMARY OF EXAMPLES Polmn. Radical Dis- Spec. Percent HF? Vapor Percent Initiator initiator: gen. rate persion melt Spec. Rel. No. dispersing prechar e density 111?]? Scheduling sol. con- (+10 solids vise. LR. Polmn.

agent (p.s.i.g5 centration moles/min l content (poises rate (molal) liter X10 1. 390 0. 235 75 Regular 0. 007 2. 14 7. 3 7. 3. 5 1. 0 2. 0 390 0.23 74 High Initial..- 0. 0096 1.24 18 7. 5 3. 5 1.2 3-.-. 0.1 390 0. 23 d 0.02 2. 6 l8 7. 5 3.5 2. 4 4- 0.2 390 0.23 0.031 4. 03 18 7. 5 3. 5 3. 7 5 0.2 390 0.23 0.02 2. 6 l8 7. 5 4.0 2. 4 6 0.2 255 0. l9 0.02 2.6 18 7. 5 3. 5 2. 4 7-... 0.1 255 0.19 0.01 1.3 18 7. 5 3. 5 1. 2 8 0. 1 418 0. 24 0. 0125 1. 63 18 7. 5 3. 5 1. 5 9- 0. 1 390 0. 23 74 Low InitiaL 0. 00174 0. 285 9. 4 120 3. 6 0. 8 10.- 0. 1 390 0. 23 74 High Initial--. 0. 0037 0.605 9. 4 120 3. 8 1. 7

EXAMPLE 1 tered by switching to the m ect1on of a freshly prepared A cylindrical, horizontally disposed, Water-jacketed, paddle-stirred, stainless steel reactor having a length to diameter ratio of about 1.5 and a Water capacity of 80.7 parts is evacuated, charged with 46 parts of demineralized water and purged of gases by Warming the charge and evacuatingthe reactor free space. The degasified charge is heated to 95 C., pressured to 390 p.s.i.g. with deoxygenated hexafluoropropylene, made 2.9 10 molal with respect to potassium persulfate by rapid addition of freshly prepared 0.011 molal solution of potassium persulfate in demineralized water, and then stirred for minutes at 95 C. At the end of the 15 minutes, the reactor is pressured to 650 p.s.i.g. with deoxygenated tetrafluoroethylene so as to achieve a mixture of comonomers which is 75 Wt. percent hexafluoropropylene and 25 Wt. percent tetrafluoroethylene. A freshly prepared 0.007 molal solution of potassium persulfate is injected at the rate of 0.0437 part per minute so that the rate of active radical generation is maintained at about 2.l4 10- moles per minute per liter of solution. The stirring of the reactor contents at 95 C. and the addition of potassium persulfate are continued for 100 minutes after the 650 p.s.i.g. total pressure is attained; during this period the pressure is maintained constant by the continuous addition of tetrafluoroethylene. At the end of 100 minutes the stirring is stopped, the vapor in the reactor is sampled, the reactor is vented and its residual contents is discharged.

There is obtained an aqueous dispersion containing 7.3 parts (about 15 Wt. percent) of resinous polymeric product. The sample of the vapor space taken from the reactor at the end of 100 minutes is immediately analyzed by infrared techniques and found to contain 75 wt. percent. hexafluoropropylene.

The aqueous dispersion is coagulated by stirring to obtain a particulate coagulum which is filtered from the liquid, Washed with distilled water and dried. This material has a specific melt viscosity of 7.5 X 10 poises, an a specific LR. ratio of 3.5.

EXAMPLE 2 A cylindrical, horizontally disposed, Water-jacketed, paddle-stirred, stainless steel reactor having a length to diameter ratio of about 1.5 and a water capacity of 80.7 parts is evacuated, charged'with 46 parts of demineralized water and purged of gases by Warming the charge [and evacuating the reactor free space The contents of the reactor is heated to 120 C. and the reactor is first pressurized to 390 p.s.i.g. with deoxygenated hexafluoropro pylene andvthen with deoxygenated tetrafluoroethylene to 600 p.s.i.g. total pressure so that the composition of the vapor space is, 74 wt. percent hexafluoropropylene and 26 Wt. percent tetrafiuoroethylene; for a period of 15 minutes during the pressure up a freshly prepared solution of 0.04 molal potassium persulfate is injected at the rate of 0.175 part per minute so that at the end of 15 minutes the calculated concentration of undecomposed persulfateis 7.85 10 molal. At the end of the 15 minutes the rate of potassium persulfate addition is al- 0.0096 molal solution of potassium persulfate at a rate of 0.0437 part per minute so that the rate of active radical generation is maintained at about 1.24 10- moles per minute per liter of solution. The stirring of the reactor contents at 120 C. and the addition of potassium persulfate are continued, for minutes after the 600 p.s.i.g. total pressure is attained; during this period the pressure is maintained constant by the continuous addition of tetraiiuoroethylene. At the end of 100 minutes the stirring is stopped, the vapor in the reactor is sampled, the reactor is vented and its residual contents is discharged.

There is obtained an aqueous dispersion containing 18 par-ts (about 37 wt. percent) of resinous polymeric product. The sample of vapor taken from the reactor at the end of 100 minutes is immediately analyzed by infrared techniques and found to contain 74 wt. percent hexafluoropropylene.

EXAMPLES 3-10 Except where the conditions are noted as being different in the Example Summary, Examples 3-10 have been carried out using the procedure of Example 2.

Although it generally has been found that the properties of the copolymers prepared by means of the present invention are the same, for any given composition and molecular Weight, as the materials prepared by processes described in the art, it has been noted that the steadystate compliance values of the products prepared by the instant process are substantially lower than those of theproducts prepared by conventional methods.

We claim:

1. 'In a process for the preparation of a coplymer of hexafluoropropylene and tetrafluoroethylene in an aqueous dispersion, said copolymer having a specific melt viscosity, as measured at 380 C. under a shear stress of 6.5 pounds persquare inch, of between 1.5 1O and 1.5 10 poises anda specific LR. ratio, as measured on film of about 0.002 inch thickness water-quenched frorn the molten product, by net absorbance at 10.18 micronswave length divided by net aborbance at 4.25 microns Wave length, in the range of 1.5 to 6, the steps which comprise polymerizing a mixture of hexafluoropropylene and tenafluoroethylene having a vapor density of 0.18- to 0.30 g./cc.'at 95 to 138 C. and a total pressure of 400 to750 p.s.i.g. in an aqueous system containinga dispersing agent having a solubility in water of at least 0.1 Wt. percent at reaction conditions and using a water soluble,.fr'ee radical-producing initiator having a half life at 95 to 138 C. of less than 9.0 minutes, said initiator being present in quantities which are at least 6.5 wt. percent in excess of that required to achieve the desired molecular weight, with the excess being contacted with the initial charge of hexafluoropropylene prior to the addition of tetrafiuoroethylene.

,2. A process according to claim 1 wherein the term penature is maintained at to 127 C.

3. A. process according to claim 1 wherein the temperature is maintained at 113 to 123 C.

4. A process according to claim 1 wherein the mixture of hexafluoropropylene and tetrafluoroethylene has a vapor density of 0.20 to 0.25 g./cc.

5. A process according to claim 1 wherein the total pressure is maintained between 550 and 625 p.s.i.g.

6. A process accordingito claim 1 wherein the mixture of hexafluoropropyiene and tetrafluoroethylene has a vapor density of 0.20 to 0.25 g./.cc. at a temperature of 113 to 123 C. and a total pressure of 550 to 625 p.s.-1.g.

7. A process according to claim 1 wherein the free radical-producing initiator has a half life at 95 to 138 C. of less than 2.0 minutes.

8. A process according to claim 1 wherein the free radical-producing initiator has a half life at 95 to 138 C. of between 0.1 and 2.0 minutes.

9. A process according to claim 1 wherein the free radical-producing initiator is added at such a rate during the polymerization stage that the rate of active radical formation is between 4 10 and 3 10- moles per minute per liter of solution at-reaction conditions.

10. A process according to claim 1 wherein the free radical producing'initiator is selected from the group consisting of ammonium pensulfate' and potassium persulfate.

5 lected from the group consisting of hydrogen and fluorine.

14. A process according to claim 1 wherein the dispensing agent is selected from the group consisting of ammonium 9'I-I-hexadecafluorononanoate and ammonium 8H-tetradecafiuorooctanoate.

15. A process according to claim 1 wherein the disper-sing agent is present in quantities 0.01 to 1.5% by Weight of the aqueous phase.

References Cited in the file of this patent UNITED STATES PATENTS 2,946,763 Bro et a1 July 26, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 132, 124 May 5, 1964 Maurice Joseph Couture et al.

v It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patentshould read as corrected below Column 3, line 61, after "the", first occurrence, insert product column 5, TABLE V, third column, line 3 thereof, for "31.4" read 3. 14 same column 5, TABLE VI, in the heading to the second major column, for "vapro read v'apor---; column 6, line 59, for "-sp'aced" read spacer columns 7 and 8, in the table, in the heading to column 8 thereof, for

"(+ 10 read lO" column 8, line 49, for "copylmer" read copolymer Signed and sealed this 3rd day of November 1964,,

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attcsting Officer Commissioner of Patents 

1. IN A PROCESS FOR THE PREPARATION OF A COPOLYMER OF HEXAFLUOROPROPYLENE AND TETRAFLUOROETHYLENE IN AN AQUEOUS DISPERSION,SAID COPOLYMER HAVING A SPECIFIC MELT VISCOSITY, AS MEASURED AT 380*C. UNDER A SHEAR STRESS OF 6.5 POUNDS PER SQUARE INCH, OF BETWEEN 1.5X10**3 AND 1.5X10**6 POISES AND A SPECIFIC I.R. RATIO, AS MEASURED ON FILM OF ABOUT 0.002 INCH THICKNESS WATER-QUENCHED FROM THE MOLTEN PRODUCT, BY NET ABSORBANCE AT 10.18 MICRONS WAVE LENGTH DIVIDED BY NET ABSORBANCE AT 4.25 MICRONS WAVE LENGTH, IN THE RANGE OF 1.5 TO 6, THE STEPS WHICH COMPRISE POLYMERIZING A MIXTURE OF HEXAFLUOROPROPYLENE AND TETRAFLUOROETHYLENE HAVING A VAPOR DENSITY OF 0.18 TO 0.30 G/CC. AT 95 TO 138*C. AND A TOTAL PRESSURE OF 400 TO 750 P.S.I.G. IN AN AQUEOUS SYSTEM CONTAINING A DISPERSING AGENT HAVING A SOLBILITY IN WATER OF AT LEAST 0.1 WT. PERCENT AT REACTION CONDITIONS AND USING A WATER SOLUBLE, FREE RADICAL-PRODUCING INITIATOR HAVING A HALF LIFE AT 95 TO 138*C. OF LESS THAN 9.0 MINUTES, SAID INITIATOR BEING PRESENT IN QUANTITIES WHICH ARE AT LEAST 6.5 WT. PERCENT IN EXCESS OF THAT REQUIRED TO ACHIEVE THE DESIRED MOLECULAR WEIGHT, WITH THE EXCESS BEING CONTACTED WITH THE INITIAL CHARGE OF HEXAFLUOROPROPYLENE PRIOR TO THE ADDITION OF TETRAFLUOROETHYLENE. 